Originally, road milling equipment was utilized to smooth out bumps in the surface of a roadway or to grind down the joinder of two adjacent concrete slabs that may have buckled. Later these road milling machines, operated with a cylindrical drum having a plurality of bit blocks mounted thereon in herringbone or spiral fashion, and bit holders with bits on top thereof in turn mounted on the bit blocks, have been utilized for completely degrading concrete and macadam roads down to their gravel base. The road milling equipment can also be used for trenching and mining operations.
Bits, such as those shown in U.S. Pat. No. 6,739,327 (327), disclose an insert having a conical cutting tip that is mounted in a recess in a frustoconical forward portion of the bit. The insert 88 is surrounded by a hardened annular collar that provides added wear resistance to the cutting tool. The tool has a solid generally cylindrical shank extending axially rearwardly from the body portion.
The bit as described in the '327 patent fits in a central bore in a bit holder as described in U.S. Pat. Nos. 6,371,567 and 6,585,326. The above-described bit holders, being frictionally seated in bores in their respective bit blocks mounted on drums, and not held therein by retaining clips or threaded nuts, provide for ease of removal and replacement when the bit holders are worn through use or broken due to the harsh road degrading environment that they are used in.
Additionally, it has been found that individual bits may wear or be broken off of their shanks because of the harsh use environment and need replacement. Historically, these bits and bit holders have been made of steel with hardened tungsten carbide tips or collars to lengthen their end use service time.
Recently, materials harder than tungsten carbide, i.e., polycrystalline diamond such as shown in U.S. Pat. No. 8,118,371 ('371), have been used in certain road milling operations, notably the degradation of asphalt layers on long roadway stretches. While the hardness of the polycrystalline diamond tip lengthens the useful life of the combined bit and bit holder shown in the '371 patent, such that the bit does not have to be removable from the bit holder, the combination includes a somewhat brittle polycrystalline diamond tip that is not suitable for use in degrading concrete highways or curved highway stretches, such as cloverleafs and the like.
A need has developed for the provision of a polycrystalline diamond structured combination bit and bit holder that is sturdy enough to withstand the forces found when degrading or breaking up the surfaces of not only macadam (asphalt) roadways but also concrete roadways.